1. Field of the Invention
The present invention relates to a circulator used in a microwave band radio device, for example in a mobile communication device such as a portable telephone.
2. Description of the Related Art
A conventional lumped element type circulator has an assembled circulator element with a circular plane shape and a basic structure as shown in an exploded oblique view of FIG. 1. In the figure, a reference numeral 10 denotes a circular substrate made of a non-magnetic material such as a glass-reinforced epoxy. Coil conductors (inner conductors) 11 and 12 are formed on top and bottom surfaces of the non-magnetic material substrate 10, respectively. These coil conductors 11 and 12 are electrically connected with each other by via holes 13 passing through the substrate 10. Circularly shaped members 14 and 15 made of a ferromagnetic material are attached to both surfaces of the non-magnetic material substrate 10 having the coil conductors 11 and 12 so that rotating RF (Radio Frequency) magnetic fluxes are induced in these ferromagnetic members 14 and 15 due to an RF power applied to the coil conductors 11 and 12. As aforementioned, the conventional circulator element in the circulator has a circular plane shape and is constructed by assembling, namely piling and bonding, the ferromagnetic members 14 and 15 on both sides of the non-magnetic material substrate 10.
The circulator is then constructed, as shown in its exploded oblique view of FIG. 2, by stacking and fixing in sequence grounding conductor electrodes 16 and 17, excitation permanent magnets 18 and 19 and a metal housing separated into upper and lower parts 20 and 21 on both ferromagnetic members 14 and 15, respectively. The housing parts 20 and 21 form a magnetic path of the magnetic flux from and to the excitation permanent magnets 18 and 19.
If an RF power is applied to the coil conductors 11 and 12 through input/output terminals not shown, RF magnetic flux rotating around the coil conductors 11 and 12 will be produced in the ferromagnetic members 14 and 15. Under this state, if a dc magnetic field perpendicular to the RF magnetic flux is applied from the permanent magnets 18 and 19, the ferromagnetic members 14 and 15 present different permeability .mu..sub.+ and .mu..sub.- depending upon the rotating sense of the RF magnetic flux, as shown in FIG. 3. A circulator utilizes this difference of the permeability depending upon the rotating sense. Namely, a propagation velocity of the RF signal in the circulator element will differ in accordance with the rotating sense and thus the signals transmitted in opposite directions will be canceled by each other resulting in preventing the propagation of the signal to a particular port. A non-propagating port is determined in accordance with its angle against a driving port due to the permeability .mu..sub.+ and .mu..sub.- of the ferromagnetic member. For example, if ports A, B and C are arranged in this order along a certain rotating sense, the port B will be determined as the non-propagating port against the driving port A and the port C will be determined as the non-propagating port against the driving port B.
The circulators have been broadly utilized as effective elements for preventing interference between amplifiers in a mobile communication device such as a portable telephone and also for protecting a power amplifier in the mobile communication device from a reflected power. With the spread of and downsizing of recent radio transmission devices, the circulators themselves are requested to be manufactured in lower cost and in smaller size and to operate with lower loss and in a broader frequency band. In order to satisfy these requirements, it will be necessary to make a circulator having a large difference between the permeability .mu..sub.+ and .mu..sub.- and having a driving circuit with small loss.
However, according to the conventional circulator shown in FIG. 1, since the driving lines 11 and 12 are formed on the non-magnetic material substrate 10 and these lines and substrate are put between the two separated ferromagnetic members 14 and 15, the magnetic path of the circulator is blocked by the non-magnetic material substrate 10. Thus, a demagnetizing field will be produced at boundary faces between the non-magnetic material substrate 10 and the ferromagnetic members 14 and 15 causing the permeability to lower. As a result, the conventional circulator cannot sufficiently satisfy the aforementioned recent requirements.
In order to obtain a compact-sized circulator by reducing the demagnetizing field produced at the boundary faces of the substrate 10 against the ferromagnetic members 14 and 15, the inventors of this application made this substrate 10 by a sheet compounding a ferromagnetic material on an experimental basis. Although, this structure can somewhat reduce the demagnetizing field at the boundary faces, it is far from satisfying the aforementioned requests.
Furthermore, since the circulator element according to the conventional circulator is made in a circular plane shape, if discrete circuit elements, such as resonating capacitors or terminations, are additionally attached to terminals on its side surfaces, a total size of the circulator will become much larger.
Also, according to the conventional circulator, since the housing which constitutes a magnetic yoke is made by mechanically combining the separated upper and lower parts 20 and 21, a magnetic resistance of the magnetic path of excitation field will become extremely high and the assembly of the circulator will become very complicated.
There are some known structures of the circulator for increasing its inductance to lower its resonance frequency, such as coil lines are wound around a ferromagnetic material member or that ribbon looped electrodes are used. However, no circulator with the former structure winding the lines around the ferromagnetic material member has been put to practical use because it is difficult to mass produce. Furthermore, although a small-sized circulator having the latter structure using the ribbon looped electrodes has been developed, this circulator has the following problems.
(1) Since the coils are open, the circulator may be easily influenced by external electoro-magnetic fields. Thus, its housing and its magnets have to be disposed apart from each other such that practical downsizing of the circulator is very difficult. PA1 (2) Since the ferromagnetic material member is prepared only at one side of the ribbon loop and thus the volume of the ferromagnetic material member will be insufficient, enough difference of the permeability .mu..sub.+ and .mu..sub.- will not be practically obtained.